Abstract. LEXEO Therapeutics, LLC, is an early stage biotechnology company focused on using in vivo gene therapy technologies to treat hereditary disorders of unmet medical need. LEXEO is developing an in vivo gene therapy strategy to mitigate the high risk for osteoporosis in individuals with aldehyde dehydro- genase 2 (ALDH2) deficiency, a hereditary disorder affecting 8% of the world population, and 35-45% of people of East Asian background. ALDH2 is a key enzyme for ethanol metabolism; with ethanol ingestion, mutations that reduce the oxidizing ability of the enzyme result in systemic accumulation of toxic acetalde- hyde. The most common variant is the ALDH2*2 allele (glutamic acid-to-lysine substitution, E487K). Heter- ozygotes have <50% ALDH2 enzymatic activity; homozygotes have <4% due to the dominant negative function of the mutant protein in the tetrameric enzyme. The combination of acetaldehyde and ethanol suppresses early osteoblast progenitor formation, leading to decreased bone formation and osteoporosis. Individuals with ALDH2 deficiency who drink alcohol have an increased risk for osteoporosis and hip frac- ture. Current forms of osteoporosis therapy have limitations in duration of effective therapy, compliance and toxicities, making the development of a specific means to prevent or reverse osteoporosis associated with ALDH2 deficiency desirable. LEXEO?s proposed therapy is a one-time intravenous administration of LEX06 (AAVrh.10hALDH2), an adeno-associated virus serotype 10 gene transfer vector expressing the normal human ALDH2 coding sequence. In collaboration with the Crystal laboratory at Weill Cornell, LEXEO has assessed osteopenia in 2 mouse models of ALDH2 deficiency. After chronic ethanol inges- tion, these models have high serum acetaldehyde levels and develop a striking osteopenia phenotype quantified by microcomputed tomography (CT) and histology with significantly lower bone volume/total volume, cortex thickness, trabecular number and thickness, and increased trabecular space. When pre- treated with intravenous administration of LEX06, there was remarkable prevention of these bone abnor- malities, demonstrating that LEX06 can prevent the development of osteopenia associated with ALDH2 deficiency and chronic ethanol ingestion. The goal of this Phase I STTR, is to demonstrate that LEX06 will also correct the osteopenia in ALDH2 deficient mice after they have been chronically administered etha- nol and have established osteopenia, documenting that LEX06 therapy can be a treatment of ALDH2 defi- cient-associated osteopenia. We propose the following. Aim 1. To evaluate the hypothesis that LEX06 (AAVrh.10hALDH2) therapy will reverse ethanol-induced osteopenia in ALDH2E487K+/+ mice. With this effi- cacy data, LEXEO will be ready to move to a phase II STTR to have a pre-IND meeting with the FDA, manufacture clinical grade LEX06, carry out formal safety/toxicology studies and submit an Investigational New Drug application to initiate a clinical trial.